Cholestatic liver diseases are highly prevalent causes of progressive liver disease in the United States and are a common indication for liver transplantation. Unfortunately, current medical therapies are not curative and may not prevent disease progression. Over the past decade, the Unfolded Protein Response (UPR), an adaptive cellular response to Endoplasmic Reticulum (ER) stress, has been implicated in the pathogenesis of many liver diseases. The role of the UPR in hepatic bile salt toxicity and cholestatic liver disease, however, remains unexplored. X-box binding protein 1-spliced (XBP1s) is a key regulatory molecule of the UPR that our preliminary data indicates is protective from bile salt injury. The central hypothesis of this proposal is that XBP1s is protective to the live during cholestasis~ and attenuated hepatic expression of XBP1s results in diminished expression of liver bile salt transporters and increased susceptibility to hepatic bile salt liver injury. Thus, in this proposal we will feed diets supplemented with cholic acid to mice and define the role of XBP1s and downstream targets in hepatic bile salt injury (Specific Aim #1). CYP7a1 is the rate-limiting step in bile salt synthesis, and inhibition of CYP7a1 expression and activity can act as a negative feedback mechanism to reduce the bile salt pool. We present preliminary data indicating that activation of the hepatic UPR markedly reduces expression of hepatic CYP7a1, likely via a XBP1s- mediated pathway. Thus, we will determine that the Unfolded Protein Response and XBP1s suppress hepatic Cyp7a1 expression, function and bile salt synthesis (Specific Aim #2). Finally, in order to directly determine that attenuation of hepatic XBP1s expression increases susceptibility to bile salt hepatotoxicity, we will develop genetically-modified mice with a liver-specific deletion of XBP1. We will develop liver-specific XBP1(-/-) mice in order to confirm the protective role, and define the protective mechanisms, of XBP1s in liver bile salt injury (Specific Aim #3). While this proposal focuses on hepatic XBP1s, the long-term goal is to develop a line of research to identify UPR signaling pathways that can serve as therapeutic targets for the treatment of cholestatic liver diseases. This proposal utilizes state-of-the-art mouse genetics, molecular biology and lipid biochemical techniques to further our understanding of the protective role of XBP1s in bile salt injury and cholestasis. Investigations identifying the role of the XBP1s signaling pathway and its effects on bile salt-induced liver injury may identify a novel pathway and potential targets for drug development to treat cholestatic liver diseases.